Pressure Washer Seals

What are pressure washer seals?

Pressure washer seals are the seals inside a pressure washer pump that stop water escaping as the pump builds pressure. The pump uses pistons, or plungers, that move back and forth to push water out at high pressure, and the seals around each plunger hold that pressure in. Over time these seals wear, and replacing them is one of the most common pressure washer repairs.

A typical pressure washer pump uses several kinds of seal:

• High-pressure seals, which seal around the plungers on the pressure side of the pump
• Low-pressure seals, which seal around the plungers on the water inlet side
• Oil seals, which keep the pump’s oil in and water out of the crankcase
• O-rings and small seals in the valves and pump head.

The plunger seals do the hardest job.

• They seal against a plunger that slides back and forth many times a second, while holding back water at very high pressure.
• Because of this, they wear gradually in normal use, and need replacing as part of routine maintenance.

Pressure washer seals are sized by their inside diameter, outside diameter, and width. Some are stepped and show two width figures, for example 18x26x6/8.

Why are pressure washer seals essential?

A pressure washer only works if the pump can build and hold pressure. Worn plunger seals let water bypass the seal, so pressure drops, the pump runs rough, and water leaks from under the pump head. If water gets past the low-pressure and oil seals into the crankcase, it can mix with the oil and damage the pump’s bearings and crankshaft.

Replacing worn seals early restores full pressure, stops leaks, and protects the rest of the pump, which is far cheaper than replacing the pump itself. Seals are available to suit popular pressure washer pumps, including a wide range of Kärcher models.

Typical applications include:

• Commercial and industrial pressure washers
• Hot water pressure washers
• Domestic pressure washers
• Car wash and valeting equipment
• Agricultural and farm cleaning equipment
• Drain cleaning and jetting equipment
• Plunger pumps used in other high-pressure water systems

The main types of pressure washer seals

• High-pressure plunger seals - Fit in the pump head around each plunger, on the pressure side. They are the seals that wear most and are most often replaced.
• Low-pressure plunger seals - Fit around the plungers on the inlet side, where the water enters the pump at low pressure. They stop water reaching the crankcase.
• Pump oil seals - Fit where the plungers leave the crankcase, keeping oil in and water out of the pump’s drive end.
• Stepped seals - Seals with a stepped profile, shown with two width figures. They are designed to fit a stepped seat in the pump head.
• Seal kits - Sets of high-pressure seals, low-pressure seals, and O-rings for a particular pump, so all the worn parts can be replaced together.

How to choose the right pressure washer seals?

• Pump make and model - Check the seal is listed as suitable for your pressure washer or pump model. The model number is usually on a label on the machine or pump.
• Size - Match the inside diameter, outside diameter, and width of the old seal. The inside diameter must suit the plunger diameter.
• Seal position - Make sure you have the right seal for the position, as high-pressure, low-pressure, and oil seals are different, even on the same plunger.
• Hot or cold water - Hot water pressure washers need seals that can cope with the higher water temperature. Check the seal suits your machine.
• Replace as a set - Replace the seals on all plungers at the same time, as they will all have worn by a similar amount.

Essential references and specialised ranges

How the numbering works:

Pressure washer seals are usually referenced by size, in mm:

Inside diameter x outside diameter x width

For example, 18x26x6/8 = 18mm inside diameter, 26mm outside diameter, with a stepped width of 6mm and 8mm.

Seals are typically made in a range of sizes to suit pump plungers, for example with inside diameters from around 12mm to 20mm and outside diameters from around 18mm to 28mm.

Seals listed for a particular pump also show the machine models they suit, so you can match the seal to your pressure washer.

Suffixes:

A slash in the width, such as 6/8, shows a stepped seal with two width figures.

Brand or range codes may be added to the end of the reference, to show which manufacturer’s pumps the seal suits.

Suffix codes vary between manufacturers, so check the supplier’s catalogue for the exact meaning.

FAQ - All you need to know about pressure washer seals

How do I know if my pressure washer seals are worn?

The most common signs are a drop in pressure, the pump pulsing or running rough, and water dripping from under the pump head. Milky or watery oil in the pump is a sign that water is getting past the seals into the crankcase.

How often should pressure washer seals be replaced?

It depends on how much the machine is used, the water quality, and whether it is run dry. Seals wear gradually, so replace them when you notice a loss of pressure or leaks, and check them during routine servicing on heavily used machines.

What damages pressure washer seals?

Running the pump without water, dirty or gritty water, hard water deposits, frost damage, and leaving the pump in bypass for long periods can all wear or damage the seals quickly.

Should I replace the plungers too?

Check the plungers when changing the seals. If they are scored, cracked, or worn, new seals will wear quickly, so replace any damaged plungers at the same time.

How do I fit pressure washer seals?

Follow the pump manufacturer’s instructions. Usually, the pump head is removed, the old seals are taken out of their seats, and the seats are cleaned. The new seals are lightly lubricated with a compatible grease and pressed squarely into place, the right way round, before the head is refitted over the plungers.

How do I identify the seal I need?

Check the make and model of your pressure washer or pump, and look for seals listed as suitable. If you have the old seal, measure its inside diameter, outside diameter, and width, and note whether it is stepped.

What are the signs that a pressure washer seal has failed?

Warning signs include:

• Low or fluctuating pressure
• Water leaking from under the pump head
• The pump pulsing, knocking, or running rough
• Milky or watery oil in the pump
• Oil leaking from the pump